Light weight panels formed of a sandwich of two facings and an intermediate "core" structure bonded together is a known structure that finds use, for example, as flooring in commercial aircraft. The sandwich panel flooring provides the high strength and stiffness and low weight characteristics desired in aircraft, for allowing greater operational efficiency. Panels of that type contain fasteners, which are also known devices, and which are affixed to the panel in a variety of ways. The fasteners permit bolts and other attaching devices to fasten the panel to support structure in the aircraft as well as to permit other structures and accessory airplane equipment to be supported on and maintained in a fixed location on the aircraft floor. A large variety of fasteners for that application has heretofore been marketed and sold by the Shur-Lok Company, Irvine, Calif., U.S.A., the assignee of the present invention.
Heretofore the sandwich panel has been constructed primarily of aluminum alloy or fiberglass reinforced plastic facings bonded to balsa wood, plastic foam, aluminum honeycomb, non-metallic honeycomb and paper honeycomb core materials. The fasteners designed for sandwich structure comprised of these materials were constructed entirely of a metal, specifically 2024-T4 or 6061-T6 aluminum alloy, possessing the light weight, strength, and formability qualities most desired in the application. More recent proposals for improvement in aircraft panel flooring specifies materials possessing greater specific strength and stiffness permitting a smaller volume or thickness of the panel, and, hence, lower weight, without sacrificing the strength and/or stiffness characteristic of the fiberglass reinforced plastic flooring which the new material is to replace. Specifically, the materials proposed are carbon/graphite fiber reinforced plastic materials. These are the materials whose efficacy in respect of enhanced operational efficiency in aircraft was demonstrated recently in the pioneering flight around the world taken by Mr. Rutan and Ms. Yeager on a single tank full of gas in the aircraft "Voyager".
All metals have a specific potential. When metals of different electrical potential are in contact in the presence of moisture, which serves as an electrolyte, a low energy electric current flows from the material of higher potential, or acting as the anode, to the material of lower potential, acting as the cathode. This is referred to as galvanic action. One result of this galvanic action is that corrosion of the metal having the higher potential, the anode, is accelerated.
Applicant has found that fasteners of aluminum, the noble material of higher electrical potential, of the kind now used in the fiberglass reinforced plastic panel flooring are subjected to galvanic corrosion and, hence, failure, when used in connection with the new panel materials, specifically the carbon/graphic fiber reinforced materials, which acts as the less noble material of lower electrical potential or cathode.
Possible solutions to this problem include adding an electrically insulated layer or coating to existing aluminum fasteners so as to electrically isolate the aluminum from the graphite. This does not seem a practical approach because of such coatings fracturing so as to expose the base metal so that it can corrode or because of problems of adhesives not adequately bonding to the coating, but its efficacy is not yet completely known. Another approach is to use a metal such as titanium or corrosion resistent steel, which have been found to be compatible with carbon graphite/fiber as a substitute for the aluminum in existing fastener structures in that those materials do not appear to corrode in that environment. The cost of titanium, however, is perhaps twelve times greater than the cost of aluminum while the weight of titanium is 1.6 times that of aluminum. Similarly, while the cost of austenitic stainless steel is approximately equal to that of aluminum, its weight is three times that of aluminum. The advantages which are obtained in using graphite fiber panels in aircraft would most certainly be traded against the very expensive titanium fasteners or the very heavy CRES steel fasteners. Because of the significant cost and/or weight increase, the substitution of titanium or corrosion resistent steel for aluminum in the fastener is not a satisfactory solution in applicants opinion.
A further solution to the problem is to substitute a plastic, specifically a thermoplastic resin including thermoplastic resins containing chopped glass or carbon/graphite fiber as a substitute for the aluminum in the existing fasteners. Although possessing compatibility with graphite fiber and being insusceptible to galvanic corrosion, the thermoplastic material does not satisfy the structural requirements imposed by the aircraft manufacturers: the fastener must withstand a significant load when applied to the floor panel spacer in the plane of the spacer flange, a shear force, and the typcial mode of failure is a tensile failure of the spacer body from the flange. The tensile strength of engineering plastics material known to applicant is approximately one-half the tensile strength of the aluminum alloy currently used in present commercially available fasteners. Although employing engineering plastics materials, the present invention does not compromise the load carrying capability of the installed fastener.
An object of the invention, therefore, is to provide a fastener compatible with carbon/graphite fiber reinforced floor paneling used in aircraft. A further object of the invention is to provide a fastener structure which avoids the serious galvanic corrosion effects found when existing aluminum fasteners are employed with graphite fiber reinforced panels. An additional object of the invention is to provide a fastener of a weight no greater than the weight of a corresponding titanium fastener. A still additional object of the invention is to provide a fastener that is of lower cost than existing aluminum fasteners so as to provide full economic benefit for the aircraft manufacturer using the new graphite fiber reinforced paneling materials as well as to provide an alternative to aircraft manufacturers who choose to continue with the existing glass fiber reinforced epoxy panels. A final object of the invention is to provide a new process as claimed for finally assembling and affixing a fastener into a honeycomb panel.